This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for METHOD FOR SYNCHRONIZING RADIO PORT AND RADIO INTERFACE IN WIRELESS LOCAL LOOP filed earlier in the Korean Industrial Property Office on Mar. 12, 1999 and there duly assigned Serial No. 36348197/1999.
1. Field of the Invention
The present invention relates to a wireless local loop (WLL) system, and more particularly to a method for synchronizing a radio port and a radio interface unit (RIU) in a WLL system, in which an asynchronous technique is used for communication by radio ports for providing timing synchronization between each radio port and each RIU associated therewith.
2. Description of the Prior Art
In a public switched telephone network (PSTN), the connection between a subscriber and an exchange is established through a local loop constructed by subscriber lines. Generally, such subscriber lines are constructed using copper lines, coaxial cables, or optical fibers. Recently, a WLL has been developed which is constructed by wireless subscriber lines using the radio wave technique.
Such wireless WLL connects the subscriber lines, which are arranged around the radio port, within the radius of about 2 to 6 Km, to the radio port in a wireless fashion. Accordingly, the WLL provides reliability and flexibility as well as economical telephone service to the public. By virtue of such features, the WLL can make it possible to provide the data communication networks in underdeveloped areas where the installation of wire telephone networks is difficult or impossible.
FIG. 1 illustrates the configuration of a typical wireless local loop that is applicable to any type of wireless system with the wide-band code division multiple access (W-CDMA) technique, using the wireless local loops as subscriber lines between a fixed network and their respective subscribers. As shown in FIG. 1, a wireless local loop includes a plurality of radio interface units (RIUs) 120 each connected to one or more subscriber equipments (facsimile machines or wire telephones); a plurality of radio ports 130 each connected to one or more RIUs 120 via radio links; a radio port controller 140 connected with the radio ports 130 via communication lines; and, a local exchange 150 connected to the radio port controller 140 via a communication line. Further, the wireless local loop includes a public switched telephone network (PSTN) adapted to connect the local exchange 150 to another network.
In a mobile communication system using the CDMA technique, a plurality of subscribers is roamed freely between the service areas defined by different base stations. For this reason, all radio ports of such mobile communication system should be accurately synchronized together, in terms of timing, in order to provide an accurate call service to a roaming subscriber without the call interruption. To this end, all base stations, based on the CDMA technique, use the global positioning system (GPS) receivers, respectively, to receive universal time from a GPS satellite for synchronization.
However, the implementation of such GPS receivers to all base stations imposes a heavy burden on the server of the mobile communication system because those GPS receivers are very expensive. Furthermore, a possible failure of the GPS satellite may cause the shutdown of all base stations.
Meanwhile, in a WLL, the subscribers (that is, subscriber equipments and associated radio interface units) are stationary, thus requiring no mobility. Accordingly, it is not necessary to achieve the synchronization among radio ports. Thus, the W-CDMA technique applied to the advanced WLLs is not required for the radio ports to use the expensive GPS receivers as the asynchronous method is used for communication by the radio ports. Accordingly, for such WLLs, it is only necessary to provide the timing synchronization between each radio port and RIU associated therewith.
Therefore, the present invention has been made in view of the above-mentioned problems involved in the related art, thus the object of the invention is to provide a method for synchronizing a radio port and an RIU in a WLL system, in which the asynchronous system is used for communication by the radio ports, capable of providing timing synchronization between each radio port and RIU associated therewith, so that the WLL system can provide inexpensive and reliable service.
In accordance with the present invention, the above object is accomplished by providing a method for synchronizing a radio port and a radio interface unit in a wireless local loop system comprising the steps of: controlling a modulator equipped in the radio port to execute the internal timing alignment thereof; controlling a channel card controller equipped in the radio port to execute the timing alignment with the modulator; controlling the channel card controller to transmit a synchronous data to the radio interface unit via the modulator; controlling the radio interface unit to acquire a pilot signal transmitted from the radio port; and synchronizing the radio interface unit with the radio port associated with the acquired pilot signal based on the synchronous data received from the radio port.
In accordance with the present invention, the radio port uses an even-second clock signal and a 20 ms clock signal to create the synchronous data for providing the timing information associated with the clock signals. The radio port then transmits the synchronous data to the radio interface unit, which in turn is synchronized with the system time of the radio port based on the synchronous data received thereto. To this end, the radio port creates the synchronous data in sync with the even-second clock signal and the 2 ms clock signal.
In a WLL system using the CDMA technique, each radio port transmits the system synchronous data to a radio interface unit associated therewith via a synchronous channel. The channel card of the radio port includes a plurality of modems each consisting of a modulator and a demodulator. The modem is controlled by a channel card controller, and the modem creates synchronous data under the control of the channel card controller.